1. Field of the Invention
The present invention relates to a semiconductor device to which a new principle of operation is applied employing the tunneling effect.
2. Description of the Related Art
As tunnel transistors employing the tunneling effect, an MIMIM structure (M: metal; I: insulator) was first proposed by Mead (Proc. IRE 48 (1960) 359). Thereafter, an MIMS structure (S: semiconductor) (Phys. Rev. Lett 6 (1961) 341) was proposed by Spratt et al. and an MIS structure (Proc. IEEE 61 (1973) 1053) by Kisaki et al. These structures have been provided as semiconductor devices in which hot electrons are employed to achieve high-speed operation and are now generally referred to as hot electron transistors (HETs).
The principles of operation of the HET comprised of the semiconductor heterojunction will hereinafter be described. HET has a semiconductor heterojunction such as the one shown in FIG. 6 and is based on the operational principle which, if a ratio of the base current and the collector current is .beta. and the base-collector voltage can be increased to more than 1/.beta. of the emitter-base voltage, an electrical power can be amplified.
To obtain a practical HET device, there are a large number of problems to be solved, as follows:
(1) Since the reflection at the collector barrier is great, there is a loss due to the base current.
(2) In some transistors (e.g., Japanese Patent Laid-Open Publication HEI 6-21435), the potential shape has been improved in order to reduce reflection at the collector barrier, but the epitaxial process is complicated.
(3) Manufacture of a very thin base and of a contact with respect to the base is difficult.
Because of the problems as described above, the amplification factor, which can be obtained with the currently reported HET, is very low. There is a THETA (Appl. Phys. Lett 64 (1994) 1138) as a HET device of the semiconductor heterojunction having a single barrier on an emitter for improvement of an amplification factor, but the aforementioned base-contact problem remains. In addition, an RHET (Jpn. J. Appl. Phys. 24 (1985) L 825) using the heterojunction double-barrier resonance tunneling effect is satisfactory in a limited use such as a switching element, but it is unsuited to a wide application such as signal amplification.